Methods to produce Tl(1223) tapes with improved properties

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Methods to produce Tl(1223) tapes with improved properties

FLUKIGER, René-Louis, GLADYSHEVSKII, Roman, BELLINGERI, Emilio

Abstract

Different methods to prepare superconducting mono- and multifilamentary Tl(1223) tapes were developed. High-purity ceramics could be produced by ex-situ or in-situ reaction under a high isostatic gas pressure. Tapes prepared by the PIT method and an in-situ reaction carried out close to the melting point of the oxide were relatively dense and textured. Interesting results were obtained for samples where part of the oxygen atoms in Tl(1223) had been substituted by fluorine. Critical current densities up to 20'000 A/cm2 (77 K, 0 T) were reached for tapes produced by the PIT method. Well textured samples were obtained by electrophoretic deposition, the deposited Tl(1223) grains being produced by a synthesis involving substantial melting.

FLUKIGER, René-Louis, GLADYSHEVSKII, Roman, BELLINGERI, Emilio. Methods to produce Tl(1223) tapes with improved properties. Journal of Superconductivity , 1998, vol. 11, no. 1, p. 23-26

DOI : 10.1023/A:1022657107312

Available at:

http://archive-ouverte.unige.ch/unige:108138

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Journal of Superconductivity, Vol. 11, No. 1, 1998

Methods to Produce Tl(1223) Tapes with Improved Properties

Rene Flukiger', Roman E. Gladyshevskii', and Emilio Bellingeri'

Received 26 May 1997

Different methods to prepare superconducting mono- and multifilamentary T1(I223) tapes were developed. High-purity ceramics could be produced by ex-situ or in-situ reaction under a high isostatic gas pressure. Tapes prepared by the PIT method and an in-situ reaction carried out close to the melting point of the oxide were relatively dense and textured. Interesting results were obtained for samples where part of the oxygen atoms in Tl(1223) had been substituted by fluorine. Critical current densities up to 20'000 A/cm2 (77 K, 0 T) were reached for tapes produced by the PIT method. Well textured samples were obtained by electrophoretic deposition, the deposited Tl(1223) grains being produced by a synthesis involving substantial melting.

KEY WORDS: Tl(1223); high-pressure synthesis, superconducting tapes; critical current density.

1. INTRODUCTION

During the last years many efforts have been invested to further develop high-T", superconductors and to make these materials more suitable for applications.

However, despite the obvious progress made over the world, conductors, especially long wires and tapes, able to operate in a regime of moderately high magnetic fields and at the temperature of liquid nitrogen, have so far not been realized. The great benefit of the high transition temperatures, intragrain critical current densities and irreversibility fields of TlBa2Ca2Cu3O9.5 and (Tl,Pb)Sr2Ca2Cu3O9_8 is evident, but applications are still limited by weak links between the grains. We report here on the preparation of superconducting Tl(1223) tapes by different methods. Our aim was to improve the grain boundaries, and by this the transport properties, by carrying out the reactions close to or above the melting point of the oxide and by introducing a high degree of texture.

2. SYNTHESIS OF Tl(1223)

We have recently developed a synthesis of Tl(1223) under high isostatic gas pressure which allows a good control of the composition by avoiding weight losses [1,2]. Precursors were prepared by mixing appropriate amounts of T12O3, TIP, PbO and Bi2O3 with SrCO3, BaCO3 and CaCO3, previously calcinated with CuO.

1 Departement de Physique de la Matiere Condensee, Universite de Geneve, 24 quai E. Anserrnet, CH-1211 Geneva 4, Switzerland

Similarly as for the synthesis at ambient pressure (see e.g. [3]), samples with a phase purity exceeding 90%

could be produced at high pressure for the nominal compositions TI07Pbo2Bi0.2Sri 8Ba<>.2Cai 9Cu3O\

[Tl,Pb,Bi(1223)] and Tl0 6Pb0 5Sri gBao^a, 9Cu3O, [TI,Pb(l223)J. The temperature regions within which pure samples are formed are relatively broad, however, the regions favorable for Tl(1223) grain growth are narrow. For example, TI,Pb,Bi(1223) high-purity powders were obtained at temperatures ranging from 920 to 960°C at a pressure of 50 bar He / I bar O2 (when not specified, the same pressure will be considered for high- pressure syntheses hereafter) for reaction times up to 3 h, and up to 980°C for slightly shorter reaction times.

Reactions performed at 985°C, where melting takes place, produced multiphase samples already after a few minutes. Grains of Tl,Pb,Bi(1223) powders prepared by a solid-state reaction had almost regular square-brick shape and a typical size of 4x4x 1.3 urn3.

In order to decrease the formation and melting temperatures of Tl(1223), substitution by fluorine was studied. Fluorine was incorporated into the phase by adding TIP to the initial precursor mixture. Pure samples could be obtained for the nominal composition Tlo.sPbo.sSri.8Bao^Ca^CujO/.. [Tl,Pb(1223)F] at 900- 960°C for reaction times shorter than 3 h [4]. The presence of F was found to enhance the grain growth.

Grains of Tl,Pb(1223)F powders prepared at 900-955°C were twice larger, typically 7x7x2 um3 bricks, than those of the corresponding F-free samples. In all samples mentioned above, including the F-free ones, the grains 23

0896-1107/98/0200-0023$ 15.00/0 © 1998 Plenum Publishing Corporation

24 Flukigcr, Gladyshevskii, and Bellingeri were stuck together, which made it difficult to introduce

texture inside the tapes by mechanical deformation without breaking the grains. This problem is aggravated by the relatively high hardness of Tl(1223) ceramics.

Samples with well separated plate-like Tl(1223) grains could be prepared by a two-step reaction involving substantial melting. For example, TI,Pb,Bi(l223) high- purity samples were produced by melting the precursor mixture at 1020-1080°C for 0.5 h, followed by quenching to room temperature, grinding and a second reaction at 900-940°C for 3 h. The main advantage of this method of synthesis is that the Tl(1223) grains in the resulting powders are larger, 10x10x1 urn3, and have a more pronounced plate-like shape.

3. FABRICATION OF TAPES

The reactions described above were carried out either in pellets (ex situ) or inside tapes (in situ). In the latter case the reaction temperature was limited by the melting point of the sheath, which could be increased to 985°C by alloying Ag with 20% Au. The phase formations, reaction kinetics and grain morphologies for ceramics prepared by in-situ reaction were similar to those observed for ex-situ reacted powders.

3.1. Powder-in-tube method

The powder-in-tube (PIT) method was used to prepare mainly monofilamentary tapes, but also tapes containing up to 259 filaments [5]. Ag or Ag,Au tubes were filled with ex-situ reacted powder or an unreacted (or previously melted, in the case of a two-step reaction) precursor mixture, respectively, and were mechanically deformed by swaging and drawing. Multifiiamentary wires were obtained by packing thin wires into tubes and repeating the deformation procedure. Tapes were produced by rolling to a thickness of 75-300 um, the final width being 2.5-3 mm. Short (3 cm) and long (up to 2 m) tapes were further studied, the former being, in the general case, uniaxialy pressed at 0.5-2 GPa. The tapes containing precursor mixtures were processed in a high- pressure furnace in order to produce the Tl(1223) phase by an in-situ reaction. All tapes were finally annealed at 850-870°C for 5 h in flowing oxygen. Fig. 1 presents photographs of transversal cross-sections of some of the wires and tapes studied here.

Efforts were made to improve the Tl(1223) grain size and texture, and by this the transport properties, by combining uniaxial pressing with an in-situ reaction (see also [6]) and by applying a high isostatic gas pressure

during the in-situ reaction. Higher density and a slight texture could be introduced this way, in particular for F-containing tapes or tapes submitted to treatments at a pressure of 2 kbar He / 1 bar O2, at temperatures slightly below the melting point of the oxide. For reaction temperatures slightly above the melting point of the ceramic, whenever colonies of large plate-like Tl(1223) grains were observed, these were systematically separated by secondary phases, misaligned with respect to each other and/or contained cracks.

Fig. 1. Transversal cross-sections of Ag-sheathed Tl,Pb,Bi(1223) wires and tapes: a 7- (diameter 1.25 mm) and a 259-filamentary (1.5 mm) wire, a 37-filamentary (thickness 140 um) tape, all prepared by the PIT method, and a 3-layer (120 um) tape prepared by electrophoretic

deposition.

3.2. Electrophoretic deposition

An alternative method to the PIT technique for tape preparation is electrophoretic deposition. This technique requires plate-like grains, a condition which is fulfilled by the Tl(1223) ceramic, as described in §2. By depositing electrophoretically Tl(1223) onto Ag bands it was possible to obtain uniform layers of controlled thickness in a simple and reproducible way. It was found that if the grains have an appropriate morphology it is possible to orient them by applying an uniaxial pressure onto a thin deposited layer [7]. Deposition and pressing can be repeated until the desired thickness is reached, optionally adding intermediate Ag layers. For example, deposition for 1 min from isobutylmethylketone (100 V), followed by pressing at 0.5 GPa and repeated from 5 to 10 times, produced well textured, 10-25 urn-thick ceramic layers. It was possible to preserve a high purity and high degree of texture after annealing up to 820°C for 5 h in flowing oxygen.

Methods to Produce Tl(1223) Tapes with Improved Properties

25

In Fig. 2, values which quantify the degree of c-axis texture, calculated from X-ray diffraction data, are plotted for different kinds of tape. For tapes prepared by the PIT method a maximum value of 60% texture was reached for in-situ reactions performed at an isostatic pressure of 2kbar. Tl,Pb,Bi(1223) powders which had been prepared by a two-step reaction could, after electrophoretic deposition, reach 95% texture for a 10 urn-thick ceramic layer. As expected, the texture decreased with increasing thickness of the superconducting layer.

Fig. 2. Texture, as determined from X-ray diffraction data, versus the thickness of the superconducting layer in substituted Tl(1223) tapes

prepared by the PIT method or by electrophoretic deposition.

4. TRANSPORT PROPERTIES

The new preparation methods exposed above allowed to reach transport critical current densities up to 20'OOOA/cm2 (77 K, 0 T) for Tl(1223) tapes. Table I summarizes reproducible transport properties observed for different tapes.

4.1. Tapes with ex-situ reacted powder

For 3 cm-long, Ag-sheathed, monofilamentary Tl,Pb,Bi(1223) tapes, prepared by the PIT method from ex-situ reacted powder, and uniaxially pressed at 1 GPa, the highest reproducible values of j

, 15'000 A/cm

, were observed for a tape thickness of 90 um. For such tapes the critical current (field parallel to the tape surface) at 0.5 T was decreased by a factor of 23 with respect to the value at 0 T. Normalized critical currents versus the magnetic field for different kinds of tape are presented in Fig. 3. Measurements of I

performed on a tape cutting off ~250 um-wide strips showed that the current was uniformly distributed inside the core. For 15 cm-long tapes, which had been rolled but not pressed, j

decreased

Table 1. Reproducible critical current densities of substituted TI(I223) tapes prepared by different methods (ex-situ or in-situ reaction, PIT

method or electrophoretic deposition, mono- or multifilamentary tape) Powder / Sheath

Preparation, Length TI,Pb,Bi(1223)/Ag

ex-situ, PIT, mono, 3cm Tl,Pb,Bi(I223)/Ag

ex-situ, PIT, mono, 2m TI,Pb,Bi(l223)/Ag

ex-situ, PIT, 37 filaments, 3cm Tl,Pb(1223)F/Ag

ex-situ, PIT, mono, 3cm Tl,Pb,Bi(l223)/Ag,Au

PIT, m-situ, mono, 3cm Tl,Pb(1223)/Ag,Au

PIT, in-situ, mono, 3cm TI,Pb(l223)F/Ag,Au

PIT,in-situ, mono, 3cm Tl,Pb,Bi(1223)/Ag

ex-situ, deposition, mono, 3cm TI,Pb,Bi(1223)/Ag

ex-situ, deposition, 3 layers,

JL [A/cm2] at 77K..OT 15'000 lO'OOO 6'500 lO'OOO 11 '000 6'000 l0'000

9'000

11'000

Ic [A]

at 77K,0T 14

7 6 8 10 6 9 1 4.5

(OT)/

Ic (0.5T) 23

- 54 24 16 12 16 29 59

to 12'OOO A/cm2, for a 2 m-long, 85 (um-thick tape to l0'000 A/cm2. Relatively low values of jc, ~6'500 A/cm2, were observed for multifilamentary Tl,Pb,Bi(1223) tapes.

The critical current density appeared to be independent of the filament thickness down to 10 um, but dropped when the filament thickness became comparable with the grain size. Multifilamentary tapes showed improved mechanical resistance and it was, for instance, possible to bend a tape to a radius of about 1.5 cm without reducing significantly its current.

Ag-sheathed monofilamentary Tl,Pb(1223) and Tl,Pb(1223)F tapes with ex-situ reacted powder showed

Fig. 3. Normalized critical current (77 K) versus the magnetic field for 3 cm-long substituted Tl(1223) tapes prepared by the PIT method or by

electrophoretic deposition.

26 Flukiger, Gladyshevskii, and Bellingeri critical current densities of 10'000 A/cm2, the field

dependence of Ic being similar to that observed for TI,Pb,Bi(1223). For none of the tapes prepared by the PIT method the ratio of the critical currents with the magnetic field parallel and perpendicular to the tape surface exceeded 2 at 0.5 T.

Samples prepared by electrophoretic deposition and uniaxial pressing have so far not been optimized with respect to the transport properties. However, reproducible critical currents of 4.5 A (77 K, 0 T), corresponding to critical current densities of 11'000 A/cm2, were measured for 3 cm-long, 120 um- thick three-layer tapes with a total thickness of the superconducting core of 30 urn.

4.2. Tapes with in-situ reacted powder

Critical current densities ranging from 6'000 to 11 '000 A/cm2 (77 K, 0 T) were obtained for 3 cm-long Ag,Au-sheathed monofilamentary tapes prepared by in-situ reaction. As for tapes prepared from ex-situ reacted powder, the highest reproducible zero field values of jc were observed for Bi-containing samples. For TI,Pb,Bi(1223) and Tl,Pb(1223) tapes heated at a pressure of 50 bar He / 1 bar O2, the critical current decreased by a factor of -18 between 0 and 0.5 T, for Tl,Pb(1223)F by a factor of 16, or even 12. The latter tapes were prepared by alternating in-situ reactions at 920-950°C and uniaxial pressing at 1 GPa, performing four cycles, the final tape thickness being 100 urn. The field dependence of Ic for TI,Pb(1223) was reduced to a factor of 12 by carrying out the in-situ reaction at 900°C and an isostatic pressure of 2 kbar. The anisotropy of the critical current with respect to an applied magnetic field (/c(B//)//c.(Bl)) for such tapes, as well as for Tl,Pb(1223)F tapes, reached 2.5 at 0.5 T. Similar transport properties were thus observed for F-free and F-containing samples, however, in the former case it was necessary to increase significantly the reaction pressure.

Despite the improvements realized for tapes with in-situ reacted powder, the transport properties are still dominated by weak links between the grains and for all tapes prepared by the PIT method an Ambegaokar- Baratoff type temperature dependence of the critical current was observed, Ic reaching at 4.2 K values about 2.5 times those at 77 K. The sharp drop of /c in a low magnetic field remained at low temperatures, however, the persistence of a critical current at high magnetic field (up to at least 5 T at 77 K and 10 T at 4.2 K), after the initial drop, indicates the existence of a few strongly coupled paths.

6. CONCLUSIONS

Several methods to produce superconducting Tl(1223) tapes, developed by us, are compared.

Reactions carried out at a high isostatic gas pressure allowed to prepare high-purity ceramics. High density and a slight texture were observed in tapes, in particular in fluorine-doped ones, prepared by the powder-in-tube method and an in-situ reaction carried out close to the melting point of Tl(1223). A high degree of c-axis texture was observed for electrophoretically deposited layers, where the ceramic had been synthesized by a two- step reaction involving substantial melting. Despite the fact that the transport properties are still dominated by weak links, some important obstacles, in particular difficulties in phase formation and in texturing, have been overcome. Our results, like recent reports on films prepared by spray pyrolysis (see e.g. [8]), show that T1(1223) remains a promising material for large scale high-Tc superconductor applications. Owing to the similarities of (Tl,Pb)Sr2Ca2Cu3O9-8 and YBa2Cu3O7.5, some of the observations made here may be extended to the latter. Experiments aiming to improve the grain links in tapes with considerable critical currents via an in-plane alignment are in progress.

ACKNOWLEDGMENTS

This work was supported by the European Community (Brite Euram Project No. 7055).

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